Here are some XHTML test cases with links to the following services:

• Web Sniffer by Mario Müller (,sniffer)
• The official W3C Validator (,w3c)
• The development version of the W3C Validator (,w3c-dev)
• The wannabe validator Validome (,validome)
• Relaxed by Petr Nalevka (,relaxed)
• Validation Service for RELAX NG by Henri Sivonen (,sivonen)
• XML schema validator by Christoph Schneegans (,sv)
• Page Valet by Nick Kew (,pagevalet)

Encoding

1. ascii-above-127

   The ASCII character encoding does not use octets above 0x7F.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

2. control-characters

   Octets between 0x80 and 0x9F do not encode printable characters in the ISO-8859-1 character encoding.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

3. http-meta-mismatch

   The character encoding declared by the HTTP Content-Type header does not match the character encoding declared by the meta element. The latter one must be ignored.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

4. http-xml-mismatch

   The character encoding declared by the HTTP Content-Type header does not match the character encoding declared by the XML declaration. According to RFC 3023, the HTTP header takes precedence.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

5. invalid-bytes-in-comment

   Opera und Firefox do not detect invalid UTF-8 inside comments.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

6. iso-bom

   An UTF-8 BOM cannot appear in an ISO-8859-1 encoded stream. At least, the character encoding is set by the HTTP Content-Type header, therefore the BOM must be decoded to .

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

7. meta-only-encoding

   The character encoding is declared only via a meta element. This is insufficient for XHTML.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

8. text-xml-without-encoding

   text/xml entities with the charset parameter omitted, XML processors must use ASCII.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

9. unknown-encoding

   There is no unknown character encoding.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

10. utf-16

    XML processors must support UTF-8 and UTF-16.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

11. utf-16-bom-with-iso-xml-decl

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

12. utf-16-without-bom

    Entities encoded in UTF-16 must begin with a BOM.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

13. utf-8-bom-with-iso-xml-decl

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

HTML compatibility

1. empty-elements

   Elements with an non-empty content model should not use an empty-element tag, and elements with an empty content model should not use an start and end tag.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

2. lang-mismatch

   lang and xml:lang attributes should be specified with the same value.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

Miscellaneous

1. malformed-doctype-in-comment

   XML parsers are required to ignore comments.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

2. missing-namespace-declaration

   Strictly conforming XHTML documents must contain a namespace declaration for the XHTML namespace.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

3. valid-html4

   This is a valid HTML 4.01 document, but not XHTML compliant.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

Validity

1. duplicate-anchors

   The name attributes of a elements and the id attributes of all elements populate the same namespace.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

2. improper-nesting-of-a-elements

   Unlike SGML DTDs, XML DTDs and schemas are unable to express element prohibitions. However, certain XHTML elements should not be nested.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

3. inline-del-block.xml

   The ins and del elements must not contain block-level content when these elements behave as inline elements.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

4. invalid-attributes

   DTDs do not provide strong datatyping.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

5. invalid-id

   1 2 3 is not an XML name.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

6. must-not-standalone

   A standalone document requires an XML document not to use entity references other than the built-in ones, not to use attributes with default values, to set all attribute values in normalized form and not to use white space in element types with element content.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

7. root-element-name-mismatch

   The name in the document type declaration must match the element type of the root element.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

8. unnormalized-id

   The ID attribute is not set in normalized form.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

9. unnormalized-id-standalone

   The ID attribute is not set in normalized form. This is not allowed in conjunction with a standalone document declaration.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

Well-Formedness

1. adjacent-attribute-specifications

   While SGML allows adjacent attribute specifications, XML requires white space there.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

2. ampersand-as-data

   & must be escaped in element content and attribute values.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

3. attrs-not-unique

   An attribute name must not appear more than once in the same tag.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

4. comment-before-xml-declaration

   If the XML declaration is present, it must appear at the very beginning of the document, i.e. it must not be preceded by a comments.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

5. early-error

   The first character already breaks well-formedness constraints.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

6. entity-without-doctype-declaration

   Entity references such as ä require a document type definition (DTD) with an definition of that entity.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

7. illegal-char-reference-1

   U+0001 is not an XML character, therefore  is an illegal reference.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

8. illegal-char-reference-110000

   U+10FFFF is the highest Unicode character, therefore � is an illegal reference.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

9. lt-in-attr

   No < must appear in attribute values.

   ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

10. malformed-cdata-section

    ]]> must not appear in a CDATA section.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

11. malformed-internal-subset

    The XML specification provides a grammar for the the internal subset, too.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

12. malformed-xml-declaration

    The XML declaration must exactly match the corresponding production in the XML 1.0 specification.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

13. unmatched-tags

    The typical well-formedness violation.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

14. whitespace-before-xml-declaration

    If the XML declaration is present, it must appear at the very beginning of the document, i.e. it must not be preceded by white space.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet

15. xml-pi

    The processing instruction target names "XML", "xml", and so on are reserved for standardization of XML.

    ,sniffer ,sv ,relaxed ,sivonen ,w3c ,w3c-dev ,validome ,pagevalet